Question

I have:

// file model.h
#include "instrument.h"
class model
{
    // A function which uses instruments and returns double.
    double value(Instrument instruments);
}

Now in file instrument.h

// file instrument.h
class Instrument
{
    // This function needs to use model.
    double value2(model* md);
}

Now in file instrument.h, should I be using #include "model.h"? That kind of seems like a bad design.

How do I design this two object Instrument and model so that they know and can use each other?

Answer 1

Forward declarations:

class Instrument;
class model
{
  // function which uses instruments and returns double
  double value(Instrument instruments);
};

//...

class model;
class Instrument
{
  // function needs to use model
  double value2(model* md); 
} 

If your class doesn’t contain data members of the other type, you don’t need the full definition of the type. For example, if you have a member pointer, function return values or, like in your case, parameters.

Also, your intuition is correct. You should keep includes in header files to a minimum. Headers should be self-contained, but not have unnecessary headers.